1. Field of the Invention
The present invention relates to a process for manufacturing an electromagnetic interference shielding metallic foil cladded plastic outer shell product, and more particularly to a process for manufacturing a plastic outer shell product cladded with a superplastic alloy foil as an outer or inner wall by combining two techniques, the superplastic forming of a superplastic alloy and the injecting of softened plastic.
2. Description of the Prior Art
In recent years, advances in technology have led to a dramatic increase in the manufacture and use of sophisticated electronic equipment. However, the highdensity electromagnetic waves produced by electronic equipment can damage or adversely affect the performance of other equipment or components. Also, exposure to electromagnetic waves is harmful to the human body. Therefore, an electrically conductive outer shell is needed to shield electromagnetic interference (EMI) produced by electronic equipment.
Heretofore, various methods have been used to shield electronic equipment. Metallic boxes and cans fabricated from steel, copper, aluminum, etc., were used to surround high EMI emitters as shielding. However, metallic shields with intricate shapes were difficult to fabricate by conventional metalworking methods. Moreover, metallic shields were cumbersome, heavy and costly. Therefore, the electronic industry resorted to metallized plating on plastics. Unfortunately, the results obtained with metallic coatings were not always satisfactory. In addition to being relatively non-economical, once such metallic coatings were scratched through, they would lose part of their shielding efficiency. Unless such conductive coatings are continuous and free of voids, electromagnetic waves will be free to pass through them. Frequently, it was difficult to obtain a dependable, 100% effective coating which was also resistant to peeling.
Further efforts by the electronics industry to develop more dependable light-weight materials for EMI shielding have led to a third approach, namely electrically conductive component-filled plastic composites. It was anticipated that intricate shapes could be molded from the composite materials by conventional means, yielding a finished part more economical and dependable than metal or metal-coated plastics.
The principle factor influencing the performance of conductive component-filled plastic composites is the aspect ratio of the conductive fillers. The aspect ratio is defined as the ratio of the maximum dimension to the minimum dimension of the filler. For example, the aspect ratio of a fiber is the ratio of the length to the diameter of the fiber. According to the electromagnetic wave percolation theory, if the conductive filler in the plastic retains a higher aspect ratio, the filler easily forms a conductive network, and thus, the critical concentration of the conductive filler required to achieve the electromagnetic shielding effect (that is, the threshold percolation concentration) is lower.
The methods for preparing conductive componentfilled plastic composites can be classified into four types. The first type involves compounding the conductive fillers in the form of powders, short fibers or flakes with the plastic matrix, and then having the mixture hot-press molded or injection molded into various kinds of plastic products for shielding EMI.
For example, U.S. Pat. No. 4,474,685 discloses a process for fabricating electromagnetic shielding products by first compounding and then molding a composite including a thermosetting resin binder and an electrically conductive filler (including carbon black, graphite and conductive metal powders). However, during the compounding with the resin matrix, the conductive powders may easily cluster, and thus are not capable of dispersing in the resin matrix. Consequently, the electromagnetic shielding efficiency of the molded products can not be effectively improved. Furthermore, since the powder filler has a lower aspect ratio, according to the electromagnetic wave percolation theory as mentioned above, the amount (i.e., threshold percolation concentration) of the powder filler added must be relatively high to achieve electrical conductivity. Consequently, the mechanical properties, color and other physical and chemical properties of the molded products are adversely affected.
On the other hand, if the conductive filler is in a higher aspect ratio form such as fibers or flakes, the filler can be loaded to a lower level. However, the cluster phenomenon is still difficult to prevent. In addition, during the compounding process, in order to maintain the original aspect ratio, the conductive filler should be strong enough to prevent breakage due to compounding. However, such a strong conductive filler is very expensive, and is thus not suitable for ordinary low cost electronic equipment. More seriously, when the resultant plastic pellets containing such conductive filler with high strength are subjected to injection molding, the mold, screws and compressing cylinder walls of the injection machine will suffer extensive wear.
The second type of method for preparing conductive component-filled plastic composites involves binding a plastic layer to enclose the conductive continuous filler by immersion or extrusion, and then cutting the conductive long fiber-filled plastic stick to a predetermined length. For example, Japanese Patent No. 60-112854 discloses a process including continuous extruding thermoplastic to enclose a copper fiber to form a round copper fiber-filled plastic stick, and then cutting the round plastic stick into pellets of a predetermined size. In order to increase the aspect ratio of the filler, the diameter of the conductive long fiber should be as small as possible. The fibrous filler must be strong enough to prevent breakage. Because the compounding step is omitted in the second method, clustering of the filler is improved. However, again, when the resultant plastic pellets containing a conductive filler with high strength are subjected to injection molding, the mold, screws and compressing cylinder walls of the injection machine will suffer extensive wear.
The third type of method for preparing conductive component-filled plastic composites, disclosed in U.S. Pat. No. 5,531,851 and German Patent DE 19517554C2, involves sandwiching an electrically conductive metal foil in between two plastic films to form a metallized laminated plastic sheet; slicing the plastic sheet into plastic strips; radially arranging the metallized plastic strips into a die of an extruder to be wetted and bound by softened plastic into a metallized plastic bar; and finally cutting the plastic bar into metallized plastic pellets of a predetermined size.
In the third method, the compounding step is omitted, thus clustering of the filler is improved. In addition, the metal foil in the plastic/metal/plastic laminated strips is reinforced by plastic. Therefore, even aluminum foil with lower strength is applicable. Hence, when the resultant plastic pellets are subjected to injection molding, the wearing of the mold, screws and compressing cylinder walls of the injection machine will be lessened. However, in the injection molding step, such aluminum foil with lower strength has a higher possibility of breaking. Therefore, there is a need to use a specially designed injection screw and injection mold.
The above three methods all involve subjecting the conductive pellets to hot-pressing or injection molding to obtain the final plastic article for shielding EMI. When injection molding is employed, in order to prevent lag, segregation, and orientional phenomena from occurring to the metallic filler in the mold, the design of the mold is very critical and complicated, thus increasing costs. Moreover, if a very thin product is desired, such as the outer shell of a notebook computer, the design of the mold is difficult, or even impossible to achieve.
In addition, floating phenomenon in the final conductive plastic article can not be prevented. Therefore, surface plastic coating on the final conductive plastic article is required, thereby increasing cost. Most importantly, according to the above three methods, the conductive filler in the plastic is not continuous (i.e. non-solid). Therefore, the resulting electromagnetic shielding effectiveness of such plastic article is limited to about 50 dB, which is far less than that of a conventional metal outer shell.
The fourth type of method for preparing conductive component-filled plastic composites applies vacuum molding technique. Japanese Patent No. 61-293827 discloses such a method, which involves introducing the molten lead/tin alloy having a low melting point into the space between two molten plastic layers; rolling it to form a plastic/metal/plastic laminated plate; and subjecting the plate to vacuum molding at a temperature higher than the melting point of the metal and the softening point of the plastic to form a predetermined shape. By employing this method, hot-pressed molding and injection molding can be omitted, and production costs can be reduced. The conductive filler inside the plastic is in a continuous form.
However, during the process for manufacturing the plastic/metal/plastic laminated plate in the fourth method, the molten metal and the plastic have different viscosity and strength. Therefore, the thickness ratio of the metal to plastic in the laminated plate is very difficult to control. More seriously, during the vacuum molding process, the metal is in the molten state. Thus it has no bonding to the two plastic plates, and thereby, separation of the laminated plate very easily occurs. Even if the separation does not occur, when the laminated plate is attached to the wall of the mold, the metal is in the molten state and is flowing. Thus, the metal liquid will accumulate at slots, holes or other concave sites of the mold, and the resulting product for shielding EMI will have defects and can even be strongly distorted.
In addition, the metal used in the fourth method has a melting point lower than the softening point of the plastic. This indicates that the final product can only be applied at a temperature lower than the melting point of the metal. Therefore, when the electronic elements including the product obtained from the fourth method are operated and generate heat in some areas, the plastic outer shell and the electronic elements will be destroyed in those areas, causing very severe adverse results.
Another serious problem common in the above four types of methods is that the surface of the metallized plastic product obtained has very little conductive material or even none. Therefore, when multiple sheets of the metallized plastic product are combined to form an outer shell of a notebook computer, cellular phone, or digital camera, there is a continuous non-conductive plastic region at the interface of two metallic plastic sheets. Thus, the combined outer shell suffers a serious wave leak phenomenon and can not achieve the required electromagnetic shielding effectiveness.
Adhering a layer of aluminum foil directly on the plastic surface is a conventional technique to solve the problem of having little conductive material on the surface of the plastic outer shell,. Although this technique is very easy, it is impossible to maintain a complete, continuous aluminum foil on the plastic outer shell of an electronic product having a very intricate shape. Wave leakage occurs on the non-continuous region; thus, the shell can not meet the electromagnetic shielding requirements.
Japanese Patent No. 4-123490 provides another technique for applying a conductive layer onto a plastic outer shell. The process involves coating a 1 .mu.m to 100 .mu.m layer of metal having a melting point of 70.degree. C. to 420.degree. C. on the surface of the mold cavity and injecting molten plastic to the mold cavity to allow the plastic to adhere to the metal layer. Although a plastic outer shell with an intricate shape can be obtained, the plastic and the metal layer have poor interfacial adherence, thus forming undesirable voids in the plastic outer shell product.
The compression casting and injection molding of magnesium alloys have drawn increasing interests in recent years. The magnesium alloy outer shell thus produced not only has high electromagnetic shielding effectiveness, but also can have an intricate shape as a plastic product. Moreover, magnesium alloy has a very low density, thus the product manufactured therefrom has the advantage of light weight. However, during the manufacturing process, protective gas should be applied to protect the magnesium alloy from ignition. Moreover, a large amount of pin holes are often present on the compression cast or injection molded product, and the magnesium alloy is easily eroded. Therefore, complicated post treatment must be employed, thus increasing the entire manufacturing cost.